1. Field of the Invention
The present invention relates to tomographic methods and particularly to the ray hardening correction in the two- or three-dimensional computer tomography.
2. Description of the Prior Art
In computer tomography, different physical effects cause artefacts in the reconstructed tomograms, which partly significantly decrease the image quality. To be able to perform measuring tasks with the desired precision and in an automated way, computer tomography reconstructions (CT reconstructions) have to be as artefact-free as possible.
Computer tomography methods are known in the art. A radiation source radiates through an object. The radiation passing through the object is weakened in its intensity depending on the length and absorption properties of the object in the optical path. A detector, which detects transmission values, i.e. the intensity of the radiation, which has passed through the object, is disposed behind the object. Typically, the detector is designed as two-dimensional pixel detector, which provides a two-dimensional transmission picture of the object on the output side, wherein the intensity of the radiation passing through the object depends both on the absorption properties of the object, which can vary over the path of the radiation through the object, and on the transmission length of the object.
Typically, an X-ray radiation source is used as radiation source. As it is known, computer tomography works on the basis of transmission images. A computer tomographic image consists of a sequence of projections, wherein the object is radiated through in a certain position, whereupon the transmission direction is altered, for example by 1 degree, to record another projection, etc. Thus, a computer tomographic image comprises a sequence of projections, wherein a rotation angle and general geometry data, respectively, are associated to every projection, wherefrom it can be derived how the position of the object has changed from one projection to the next. Additionally, every projection comprises a two-dimensional array of transmission values, which are typically intensity values.
Depending on the desired application, for example 360 projections can be recorded, when the object is always rotated by 1 degree between two projections. Depending on the application case, however, significantly more or significantly less projections are possible. The individual projection are then, as it is known, processed with reconstruction methods, such as filtered reprojection, to generate three-dimensional volume data, which consist of a plurality of volume elements or voxels. Typically, in a three-dimensional computer tomography, a value is associated to every voxel, from which can be seen which absorption density the respective element has at this location.
The three-dimensional computer tomography is not only applied in the medical field, but particularly in the industrial quality control of devices under tests with regard to quantitative statements, such as measuring tasks. One of the most import application cases is the production of cast parts in the automobile industry. The quality control of cast parts comprises mainly finding of defects and testing of dimensions. Main tasks in the preseries development are the fast checking of the dimensional stability of cast parts with complex geometry as well as the analysis of deviations of the geometry data from required data contained in a part plan.
Under the aspect of industrial applicability in comparison to other sources (synchrotron or gamma radiator) X-ray tubes are preferably used as radiation sources. Instead of a line detector in the two-dimensional computer tomography, a flat X-ray detector is used in the three-dimensional computer tomography. The three-dimensional computer tomography requires only one rotation of the object for reconstruction, whereby measuring times are significantly reduced compared to two-dimensional computer tomography.
However, the X-ray tubes used in computer tomography emit polychromatic radiation. The interaction of the polychromatic X-ray radiation in the transmission through the matter is energy-dependent. Real system characteristic curves, which associate an intensity value to a transmission length, have thus a nonlinear curve, caused by effects like ray hardening, compton scattering and nonlinearities of the detector. This causes artefacts, such as stripes, unsharp edges, ton shaped drawings and so-called cupping effects in the reconstructed object illustration, which decrease the image quality and impede or even prevent measuring tasks.
A simple and common correction method in computer tomography is the usage of a prefilter, which operates as frequency high pass filter. Thereby, the energy spectrum of the X-ray radiator is limited to higher energies. More costly correction methods determine the nonlinear curve of the characteristic curve by measurements at reference objects from the same material as the object under test, wherein step wedges are preferred. A step wedge consists of portions of different thickness, wherein the respective thickness of the portions is exactly known. A projection of the step wedge provides a transmission value for every known thickness, so that the system characteristic curve, which indicates the connection between transmission value (intensity or weakening reciprocal to intensity with reference to a reference intensity value) and transmission length, can be generated by the reference object. In this system characteristic curve, which is generated by the reference object, influences of both the radiator and the detector are taken into consideration.
After a calibration method for determining the system characteristic curve of a radiator/detector system by using the reference object, the established system characteristic curve will then be stored and used in a subsequent measuring process to subject projection data generated from a object under test to a precorrection, to perform a three-dimensional reconstruction based on the projection data precorrected according to the system characteristic curve, in order to generate volume data, based on which the density of the object under test can be read in dependency on the position in the object under test.
The reference object method is disadvantageous in such that first a reference object has to be introduced into the system of radiator and detector to calibrate the system. Above that, a defined reference object is required, whose density is specified. If, however, the material density of the objects under test deviates from the density of the reference object, the system characteristic curve might no longer be correct.
In the article for the annual meeting 1998 of the German society for destruction free testing (Deutsche Gesellschaft für eine zerstörungsfreie Prüfung), Bamberg, Sep. 7 to 9, 1998, entitled “Korrektur der Strahlaufhärtung in der Computertomographie unter Verwendung simulierter und realer Objektdaten”, O. Haase et al. a method is sketched, which utilizes knowledge about geometry and material composition of the object under test to enable a ray hardening correction without test body. The geometry data of the object are either determined by simulation of the real object or from the reconstructed image matrix itself. For a predetermined material composition, the weakening in the object is calculated and thus, the measured weakening values are corrected. The calculation of the weakening is performed with a Monte-Carlo method, the EGS4 code, wherein both the energy spectrum of the utilized X-ray radiator and the properties of the detector are taken into consideration. The weakening coefficients of the elements are taken from literature tables.
This concept for ray hardening correction does not require a reference object, but predetermined material compositions, the spectrum of the utilized X-ray radiator and the properties of the detector as well as weakening coefficients of elements from literature tables. This leads to the fact that the method does not have the required flexibility in certain applications, namely when not all required information is present.